Conventional engines in vehicles not only provide power for the drivetrain but also provide power for accessories. Such accessories may include an air conditioning compressor, a water pump, a power steering pump, an alternator, etc. Generally, these accessories are driven by the engine via driven pulleys coupled to a driving pulley by a belt, with the driving pulley driven by the engine crankshaft. Thus, the accessories are driven at some predetermined ratio of the engine crankshaft speed, with the driving speed changing when the engine speed changes.
In today's vehicles, drivers expect that the accessories will work for all engine operating conditions. Thus, at engine idle, when the engine is generally running its slowest, and therefore the accessories are driven at their slowest speeds, the ratio of accessory driven speed to crankshaft speed must be sufficient to completely power these accessories (i.e., have adequate capacity). However, with this capacity designed-in for low engine speed conditions, when the engine is running at very high speeds (RPMs) the driving speed may be too high and overpower some accessories, creating the possibility for excessive wear of the accessories and additional noise and vibrations. Currently, for a given set of accessories, this situation requires a trade-off, then, between the minimum idle speed allowed for an engine and the maximum speed at which the engine can operate depending upon the ratio of the pulleys.
With a fixed pulley system and accessory size, possible solutions to this predicament are to either limit the minimum RPMs for engine idle conditions, thereby allowing for adequate capacity for the accessories, or to limit the upper speed (RPM) range, limiting the potential for overpowering; neither one a very satisfactory solution. The first hurts fuel economy at idle and the second would limit the engine power.
Accordingly, it is desirable to provide an accessory pulley system that will allow the accessories to be sized to handle the accessory load (adequate capacity) at low idle speeds, to increase fuel economy, while not over-driving the accessories at high engine speeds and minimizing the cost and complications needed in the pulley system to accomplish this. Furthermore, it is desirable that this system will operate automatically, without the need for external inputs, in order to minimize the complexity of the system.